Various types of implantable medical devices are in use for dispensing medication within the body. These devices either have reservoirs which are to be filled for dispensation on a time-release basis, such as an implantable drug dispenser, or have ports for insertion of medication which is dispensed through an implantable catheter, commonly known as a catheter access port. In these devices, the reservoir for receiving medication is commonly sealed with a pierceable septum. A hypodermic needle is inserted through the skin and through the septum into the reservoir. Once within the reservoir, the medication is dispensed from the syringe.
It is critical to the performance of this process that the needle tip is properly positioned at the desired dispensing location. If the needle is outside the device, medication will improperly be dispensed to the body. If the needle opening is within the septum, excess pressure in the syringe may be required to dispense medication or the dispensing may be entirely prevented. Techniques have been tried to sense needle placement involving complex apparatus for determining needle placement. What is needed is self-contained apparatus for sensing when the needle has reached the bottom of the reservoir, which does not require intricate equipment.